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We discuss medium access control (MAC) issues related to the migration to the next generation on-demand mode for communication support of NASA missions in near Earth orbits. For this scenario, to meet the objective of bandwidth-efficient support while guaranteeing specific QoS requirements, a detailed investigation of the suitability of the MAC schemes is performed. Performance measures of interest include end-to-end delay and successful throughput. A reservation-based demand time division multiple access (RD-TDMA) protocol is proposed and its performance is analyzed. Using simulation, we compare the system's performance under this scheme with that obtained under a fixed-assignment scheme. Simulation results demonstrate that on-demand mode is a suitable strategy for future NASA space missions with an unpredictable traffic pattern, and can offer certain performance advantages.